Coupling energy from electromagnetic radiation in the frequency range from about 0.1 terahertz (THz) (3000 microns) to about 7 petahertz (PHz) (0.4 nanometers), referred to as the terahertz portion of the electromagnetic spectrum, is finding use in numerous new applications. These applications include improved detection of concealed weapons and explosives, forensics, improved medical imaging, detection of biological materials, better characterization of semiconductors; and broadening the available bandwidth for wireless communications.
In solid materials the detection of electromagnetic radiation starts with absorption, which is the mechanism for transferring energy from an electromagnetic (EM) wave to an electron-hole pair. In particular, photoconductor semiconductor devices use the absorption mechanism on receiving the EM wave and transfer the received energy via electron-hole pairs by band-to-band transitions. In addition, extrinsic photoconductor devices use the absorption mechanism and operate having transitions across the forbidden-gap energy levels (S. M., Sze, “Semiconductor Devices Physics and Technology”, 2002, page 285).
Photodetectors include a range of semiconductor devices. These devices can include various types of photodiodes such as heterojunction, avalanche, P-I-N, and the like. The absorption coefficient is a property of a material and defines the extent to which the material absorbs energy in the form of electromagnetic radiation. Cut-off wavelength is the wavelength below which a material normally does not absorb electromagnetic radiation. Representative semiconductor materials such as Silicon(Si), Germanium (Ge) and Gallium Arsenide (GaAs) have cut-off wavelengths of about 1.1 microns, 1.9 microns and 0.87 microns, respectively. Hence, one particular semiconductor material normally cannot absorb energy in both the visible (i.e., about 0.39 microns to about 0.77 microns) and the infrared (i.e., about 0.77 microns to about 1 millimeter) portions of the electromagnetic spectrum.
In a given metal the electron density is normally substantially uniform. Variation or modulation of the electron density is referred to as a charge density wave. Plasmons are a form of the charge density waves. By one definition, the particle name for the charge density wave is a plasmon. A particular type of plasmon typically occurs at an interface between a metal and a dielectric, or between a semiconductor and a dielectric, and is referred to as a surface plasmon. Measurement of features on a surface at ultra-high sensitivity can employ the use of surface plasmons. For example, the technology for measuring a microbe or a virus has recently developed through the use of surface plasmon detection.
One method, called the minimum reflection method, for detecting surface plasmons includes directing an electromagnetic wave at an angle incident to a dielectric-metal or dielectric-semiconductor interface. Generally, the EM wave is reflected off the dielectric-metal interface. As the angle of the incident EM wave is varied, a particular angle is reached where the reflected EM wave is substantially zero. At this particular angle, energy of the incident EM wave is generally transferred to the surface plasmons or plasmons. Hence, the angle at which the reflected EM wave is minimum indicates the detection of plasmons. A so-called Kretschmann-Raether configuration applies the minimum reflection method (above) and provides easy access by employing a prism that contacts a metal or semiconductor layer. An electromagnetic wave passes through the prism and can reflect off the layer. An Otto arrangement disposes a prism a distance from an interface of the metal or semiconductor layer and detects plasmons again using the minimum reflection technique. This arrangement presents a disadvantage, because the interface is difficult to access with the detector. In yet another configuration, a corrugated surface or grating can be used to detect the minimum reflection of the EM wave. Another method for detecting plasmons collects an image of the reflected EM wave. The image can be processed using digital signal processing (DSP) to provide an angle of resonance within a few microns. This method is generally costly. In U.S. Pat. No. 5,792,667, plasmons are detected by measuring a temperature rise on the metal or semiconductor layer by using an ultra-thin-film thermometer. This method has the disadvantages of requiring ideal temperature control and precise calibration of the thermometer.
We describe a structure for receiving electromagnetic radiation, stimulating plasmons and generating a current on detecting the plasmons. This structure can be used as a plasmon detector. Optionally, the structure can be used to detect electromagnetic radiation over a broader range than any particular semiconductor detector. A plasmon source can be formed within a semiconductor device, such as a diode or transistor with a P-N junction. The plasmon source can include a transmission line, a microstructure, a micro-resonant structure having a cavity, a portion of metallization within a microcircuit, and the like. An electromagnetic wave can be received at the plasmon source, thereby stimulating plasmons. Fields are generated by the stimulated plasmons and coupled near the junction. The fields interact with a built-in electric field that typically occurs across the junction. This changes the band-gap and enables a current to couple through the structure.